A large number of welders that employ electromagnetic switches for turning on/off the input power thereto have been commercialized. The electromagnetic switch (hereinafter called "EM switch") can save waste power when the welder is not operated but at rest. The waste power is supposed to supply an exciting current to a main transformer and the like although the welder is at rest.
A conventional welder employing an EM switch that turns on/off the input power thereof is described with reference to FIG. 6. FIG. 6 illustrates an essential part of a circuit that controls turning on/off of an EM switch, and FIG. 6 also shows connections between the circuit and the main transformer for welding, as well as the EM switch respectively.
In FIG. 6, a main transformer 1 of the welder is coupled to three input terminals R, S and T of three-phase alternate current via three contacts MSa of an EM switch. The two input terminals (S and T in FIG. 6) are connected to a power control switch 2. A control circuit of the EM switch is formed on a secondary side of the power control switch 2. Between the two bus lines 3 and 4 of the secondary side of the power control switch 2, a contact Ta of an off-delay timer T and a coil of the EM switch MS are connected in series. A coil of the off-delay timer T and a contact CR1a-1 of a control relay CR are connected in series also between the two bus lines 3 and 4. Further, independent of this circuit, a welding control circuit 9 is formed by connecting a contact of a starter switch TS, a coil of a control relay CR1 and a power supply 8 in series.
Operations of these circuits shown in FIG. 6 are described hereinafter. When the starter switch TS is turned on, the control relay CR1 is energized and turned on. (The starter switch is, in general, mounted to a welding torch as a trigger or the like, and is thus called a torch switch.) Since the contact CR1a-1 of the control relay CR1 is serially connected to the coil of the off-delay timer T, which is energized and turned on, then the contact Ta of the timer T is closed, whereby the coil of the EM switch MS is energized and turned on. The three contacts MSa are closed in unison to supply power to the main transformer of the welder to start welding. When the starter switch TS is turned off, the coil CR1 is deenergized, and its contact CR1a-1 is opened immediately, which deenrgizes the coil of off-delay timer T. However, the contact Ta of off-delay imer T is characterized by a delay at opening (OFF), thus the contact Ta is opened after a given time, and deenergizes the coil of the EM switch MS.
Then, the three contacts MSa are opened immediately and cut off supplying power to the main transformer.
As such, the EM switch MS is automatically deenergized after a given time. A circuit incorporating this function is called, in general, an energy saving circuit (the circuit "6" in FIG. 6). The turn-off of EM switch MS is delayed from that of starter switch TS. If this delay following the turn-off of starter switch TS is not available, the EM switch MS could be turned off while welding arc still remains. If this turn-off happens while welding arc still remains, the contact MSa cuts off a large amount of current, thereby to shorten the life of contact MSa. Therefore, as detailed above, the EM switch MS is turned off after a given time of the turn-off of switch TS. This delay time takes generally several minutes. The circuit constructed as above works well as far as a normal voltage is applied to the coil of the EM switch MS; however, when the voltage applied to the coil is abnormally lowered with some reason, the coil of EM switch sometimes encounters being burnt out.
FIG. 7 illustrates one of the reasons why the applied voltage is lowered abnormally. In FIG. 7, a three-phase parallel load 10 besides the main transformer is connected to the power supply. The coil of the EM switch MS is connected between R phase and S phase. If the S phase in this three-phase power supply becomes an "open phase", no voltage is supplied from S phases. However, a detour circuit shown with a heavy line in FIG. 7 is formed, and this circuit applies a voltage between R and S phases by connecting the coil of EM switch MS with one of the phases of the parallel load 10 serially. A voltage applied across R phase to T phase is split according to an inner impedance ratio of the EM switch coil vs. one phase of parallel load 10. The voltage applied to the EM switch coil is abnormally low because one of these split voltages is applied to this coil.
FIG. 8 illustrates a relation between the voltage applied to the EM switch coil and the current running through this coil. When the voltage (hereinafter called "coil voltage") increases, the current (hereinafter called "coil current") also increases. The EM switch MS turns on not before the coil voltage reaches the turn-on voltage "Eon", and then the contact MSa is turned on. Once the EM switch MS turns on, the coil current decreases sharply, however, the coil current just before the EM switch MS turns on reaches to as much as several times of the rated current value "P" (approximately 7 times in the example shown in FIG. 8.) Therefore, as shown in FIG. 8, when a normal voltage is not applied to the EM switch coil due to the open phase of "S" phase, the coil voltage cannot reach to the turn-on voltage "Eon". The current as much as several times of the rated current value "P" is thus kept running through the coil while the EM switch MS is left not being turned on. When such an abnormally large coil-current is kept running, the EM switch coil starts being burnt in several minutes or so.